The present invention relates generally to current references, and more specifically to current references that provide substantially constant current.
Current references are circuits that are designed to provide constant current. The constant current is utilized in other circuits, and the design of these other circuits typically relies on the current being constant. One problem with current references is that the current provided can be sensitive to voltage, temperature, and process variations. That is to say, as the voltage, temperature, or process parameters (such as transistor threshold voltages) vary, the current generated by the current reference also varies.
A prior art current reference that is process and temperature dependent is shown in FIG. 21.7 of R. Jacob Baker et al., xe2x80x9cCMOS Circuit Design, Layout, and Simulation,xe2x80x9d (IEEE Press 1998). As described, the accuracy of the reference current of this circuit is limited by the threshold voltage accuracy (process dependent); and the reference current has a large negative temperature coefficient (temperature dependent).
Sensitivity to temperature and power supply voltage variations in current references, and the reduction thereof, has been the subject of much study. See, for example, Sueng-Hoon Lee and Yong Jee, xe2x80x9cA Temperature and Supply-Voltage Insensitive CMOS Current Reference,xe2x80x9d IEICE Trans. Electron., Vol.E82-C, No.8 August 1999.
Sensitivity to process variations has been historically handled by design margins. For example, if, over expected process variations, a current generated by a current reference can vary by a factor of two, the current reference is typically designed to have a nominal current equal to twice the minimum specified value so that under worst case conditions, the minimum current value is guaranteed to exist. Power is wasted as a result, in part because the nominal current value is twice what is needed.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a current reference with reduced sensitivity to voltage, temperature, and process variations.